Molecular sieves have been used to separate distinct classes of organic compounds and have also been used to separate compounds within a given class. The separation of n-paraffins from branched paraffins with 5A molecular sieves is well known. Selective adsorption of aromatics from mixed streams with 10X and 13X sieves is also known. The use of 10X molecular sieves to separate mixtures of aromatics has been disclosed in U.S. Pat. No. 3,114,782 issued Dec. 17, 1963 to Fleck et al. and U.S. Pat. No. 3,133,126 issued May 12, 1964 to Fleck et al. These patents disclose separations of mixtures of monocyclic aromatics and separation of mixtures of dicyclic aromatics. U.S. Pat. No. 3,558,732 issued Jan. 26, 1971, U.S. Pat. No. 3,626,020 issued Dec. 7, 1971 to Neuzil, and U.S. Pat. No. 3,665,046 issued May 23, 1972 to De Rosset, deal with the use of Type X and Y zeolites for separation of a C.sub.8 aromatic isomer (e.g., p-xylene) from mixtures of such isomers.
None of the above cited patents disclose that the water content of the zeolite (which depends on the conditions of the dehydration or activation process) can be critical in determining selectivity of the adsorbent.